The present invention relates to wideband frequency conversion or heterodyne radio frequency receivers, and more particularly to overload distortion protection for such receivers which are part of a measurement instrument.
A heterodyne receiver which is used as part of a measurement instrument often requires very high dynamic range. Such a receiver typically has a wide bandwidth variable gain input stage followed by a frequency conversion stage to produce a band-limited intermediate frequency signal, and the filtering imposed on the intermediate frequency signal generally removes unwanted signals and focuses on the signal of interest. The dynamic range of the receiver is defined as the separation in amplitude between the signal of interest and any interference. At low signal strength the dynamic range is limited by the receiver""s noise figure. At high signal strength the dynamic range is limited by a third-order intercept (TIO) of the receiver. A high dynamic range requirement puts stress on the frequency conversion stage, as it must contend with all signal power present in the full bandwidth input. Measurement instruments typically have an input range of several hundred megahertz up to many gigahertz of frequency span. When used for testing operational gear, this means that the instrument may be exposed to any radio signal in the environmentxe2x80x94amplitude modulation (AM) and frequency modulation (FM) broadcast radio, very high frequency (VHF) and ultra-high frequency (UHF) National Television Standards Committee (NTSC) television broadcast, VHF and UHF 8-VSB (vestigial sideband) digital television broadcast, pagers, advanced mobile phone system (AMPS), time division multiple access (TDMA), coded division multiple access (CDMA) and general service mobile (GSM) cellular radio signals, cellular digital packet data (CDPD) modem carriers and others. Each signal type has a different modulation scheme and a different peak-to-average ratio. A signal with a higher peak-to-average ratio may cause intermodulation distortion in a receiver at a lower average power level than a continuous wave (CW) signal of the same total power would.
Optimizing dynamic range requires careful planning and control of the signal levels throughout the receiver, and especially those applied in the frequency conversion mixer. Gain through the receiver is typically adjusted to control the level of desired signal at the final detector. This gain adjustment often only takes into account the band-limited IF signals as measured at the IF detector. The result is that intermodulation distortion caused by a less well controlled wide spectrum of excess power applied to the input, and before the narrow IF filtering, may cause errors in the measurement of the signal of interest.
Any spectrum analyzer has to deal with this problem. Previous solutions have included preselection filtering of the input bandwidth into frequency sub-bands, thereby limiting the total power presented to the first mixer. This solution may involve either a bank of switch-selected filters or a single tunable filter. Preselection filtering may be expensive, both in terms of the components involved as well as the required circuit board area. A skilled user of a spectrum analyzer might also recognize the signs of receiver generated intermodulation in the display and increase the radio frequency (RF) attenuation accordingly. For an instrument that displays measurement results in other than spectral form, such as error vector magnitude (EVM) or code domain power on a CDMA signal, any receiver generated distortion is difficult, if not impossible, to discover from the results alone.
Another solution by Qualcomm Incorporated is disclosed in U.S. Pat. No. 5,930,692 xe2x80x9cMethod and Apparatus for Increasing Receiver Immunity to Interferencexe2x80x9d. The power level of a received signal is detected and, if the power level meets or exceeds a predetermined power threshold, the effective gain of a low noise amplifier is decreased by inserting attenuation, either continuously or in steps, or by bypassing the low noise amplifier. This does not address the problem of high peak-to-average signals giving low readings of RF power using a logarithmic detector, i.e., it is sensitive to the type of modulation on any potential interfering signal. Also this does not optimize the dynamic range by controlling both the wideband and intermediate gains. Finally there is no indication of a warning when, despite attempts to balance the dynamic range, there still is the probability that there are distortion products in the signal display.
What is desired is an alternate approach to solving the problem of out-of-band signals, regardless of modulation type, generating distortion in an instrumentation receiver.
Accordingly the present invention provides overload distortion protection for a wideband receiver by detecting the signal power output from a wide bandwidth low noise variable gain input amplifier, the wideband detector including a peak detector. The detected signal power and an IF signal power at the output of the wideband receiver are processed by a controller to determine appropriate gain to be applied to the variable gain input amplifier and an IF amplifier to achieve wide dynamic range without intermodulation distortion. The controller sets the gain of the variable gain input amplifier to provide a nominal amplitude of the input RF signal to a frequency gain stage, and sets the gain for the IF amplifier to provide a maximum amplitude value that uses the full scale for an AND converter. Periodically the controller polls the RF and IF powers and compares the two to determine an optimum gain for the variable gain input and IF amplifiers, or indicates an error in the measurement if the input signal is so large as to result in distortion in the output measurement.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing figures.